(1) Field of the Invention
The present invention relates to a synchronizing signal switching system for synchronizing operations of a plurality of disk devices in accordance with a master synchronizing signal, and more particularly to a synchronizing signal switching system that automatically switches a supply of, a master synchronizing signal from a particular disk device to another disk device, when the particular disk device stops to provide the master synchronizing signal or when the frequency of synchronizing signals contained in the master synchronizing signal deviate from a standard.
In a computer system connected to a plurality of disk devices for external storage, operations such as parallel data transferring operations of the disk devices must be synchronized. Namely, a plurality of disk devices are controlled by a host controller so as to synchronize not only the rotating speed of all of the disk devices but also the phases of the sectors in all of the disk mediums in the disk devices, for effecting parallel data read/write operations on the plurality of disk devices. To effect the synchronization, a particular one of the disk devices is preselected as a master synchronizing signal generating source, i.e., a master disk device, by supplying a control signal from the host controller or by manually setting switches provided on the disk devices. The thus preselected master disk device generates a master synchronizing signal with at least one index pulse corresponding to one rotation of the disk medium. The master synchronizing signal is supplied to all other disk devices for the synchronization.
When the master synchronizing signal is stopped or distored during working of the system due to some accident or problem, the system must conventionally be stopped in operation, and another disk device must be set as a new master synchronizing signal generating source. It is desirable, however, to continuously supply the master synchronizing signal without stopping the system.
(2) Description of the Related Art
Conventionally, a computer system, which involves a plurality of disk devices for external storages, employs a synchronizing signal to synchronize the rotation speeds of spindles of the disk devices and to synchronize the phases of sectors provided on the disk mediums to effect parallel data transfer. Namely, a master synchronizing signal is employed for synchronously controlling the rotation speeds of spindle motors of the disk devices and for coinciding the sectors on the disk mediums in different disk devices so as to enable parallel read/write operations on the plurality of disk devices. To this end, a host controller for the external storage devices includes a master synchronizing signal generating circuit from which the master synchronizing signal is supplied to the other disk devices. Alternatively, each of the disk devices may have a master pulse generating circuit, and a selected one of the disk devices is determined as a master pulse generating disk device by manually setting dip switches mounted on the disk devices, and in this case, the master synchronizing signal generated from the selected one of the disk devices is supplied to the other disk devices.
In such a conventional method to supply the master synchronizing signal, however, the disk devices and the sectors therein will not be synchronized once the master synchronizing signal is stopped or the master synchronizing signal is distored or deviated from the standard for some reason. If the stoppage or the distortion of the master synchronizing signal happens, the system operation must conventionally be stopped, and another disk device must be determined as a new master pulse generating disk device, or the master pulse generating circuit in the host controller must be replaced by a new one. Such stoppage of the system operation is a problem in the conventional art.